1. Field
The present invention relates to wireless communications.
2. Background
A modern-day communications system may be required to support a variety of applications. For example, a system may allow for voice and data communications between users over a terrestrial link. One such communications system is a code division multiple-access (CDMA) system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as the IS-95 standard. Another such system is a CDMA system that conforms to the six-part TIA/EIA/IS-2000 series, also known as 1x. Another CDMA system that supports high-data-rate communications conforms to the high rate packet data air interface specification “TIA/EIA/IS-856”, also known as 1x Evolution—Data Optimized (1xEV—DO).
A network that supports wireless data communications serves data wirelessly to multiple users. In some examples, the network may serve data to one user at a time. Some users may observe a transmission channel of good quality, such that they may receive data at a high rate. Other users may observe a channel of poor quality, such that they may receive data only at a lower rate. The observed quality of a transmission channel may change over time, such that a user may receive data at different rates over corresponding different periods of time.
Two parameters that may be used to measure the performance of a data communications system are (1) the transmission delay required for transferring a data packet and (2) the average throughput of the system. Transmission delay does not generally have the same impact in data communications as it does for voice communications, but nevertheless it may be an important metric for measuring the performance of a data communications system. For example, a lower transmission delay may lead to a greater potential for using real-time applications. The average throughput may serve as a measure of the efficiency of the data transmission capability of the communications system.
It is desirable to maximize system throughput. This criterion suggests that at any one time, the user observing the best channel quality should be served. However, it is also desirable to be fair, so that a user observing a poor channel quality will still be served occasionally.
Unfortunately, serving a user at a low data rate may be detrimental to total system throughput. In a case where the network serves data to one user at a time, serving a user at a low data rate may prevent the network from serving more data to another user over the same period of time (i.e. at a higher data rate). Therefore, while it is desirable to serve all users waiting for data at least occasionally, it is also desirable to serve each user at a high data rate.